fix(ignore_sha): change default from false to 'false'

This was noted by @Gisleburt here styfle#59 (comment)

According to the spec the parameter should be a string, but if you use false without
quotes in YAML it is taken as a boolean so the types are not quite correct

action.yml

@@ -11,7 +11,7 @@ inputs:
   ignore_sha:
     description: 'Optional - Allow canceling other workflows with the same SHA. Useful for the `pull_request.closed` event.'
     required: false
-    default: false
+    default: 'false'
   access_token:
     description: 'Your GitHub Access Token, defaults to: {{ github.token }}'
     default: '${{ github.token }}'